This invention relates to a digital signal combining or switching circuit for use in a diversity receiver for digital communication, in which two or more carrier signals modulated by a common digital modulating signal are transmitted from a transmitter and received by the receiver through a plurality of propagation routes, paths, or channels, respectively.
The above-mentioned modulating signal may be a multipliexed signal such as, for example, a multichannel PCM signal. Various techniques for diversity communication, such as space diversity and frequency diversity, are resorted to in order to raise the reliability of radio communication against fading. In space diversity, the modulated carrier signals mentioned above are those transmitted from a plurality of antennas, respectively. Merely for brevity of description, a receiver of the type specified will be called a "digital" diversity receiver herein. Also, the modulated carrier signals will be referred to as "route" signals although this invention is not restricted to a digital signal combining circuit for use only in a receiver for route diversity communication.
In any of the diversity communication techniques, the route signals are demodulated in the receiver into intermediate frequency signals, respectively. Each intermediate frequency signal is further demodulated into a baseband signal. A receiver output signal exempted from adverse effects of fading is produced by combining either the intermediate frequency signals or the baseband signals. In digital communication, the baseband signals, namely, sequences of route digital signals as called herein, are more convenient for this purpose than the intermediate frequency signals. A digitial signal combining circuit therefore produces the receiver output signal by selecting the digital signal sequence derived from one of the route signals that is indicated by a selection signal to be least subjected, of all the route signals, to fading. In other words, the digital signal combining circuit switches, in response to the selection signal, a digital signal sequence derived from a first of the route signals to that derived from a second as soon as the second route signal becomes less subjected to fading than the first.
The route signals reach the receiver with propagation delays, which may be different from each other to give rise to a phase difference between two of the route signals. The propagation delays and hence the phase difference or differences are liable to fluctuate relative to each other as a result of fading. In digital communication, particularly in high-speed digital communication, the phase difference results in instantaneous loss or duplication of one or more bits in the receiver output signal each time when the digital signal sequences are switches from a first to a second. The loss or the duplication gives rise not only to a code error in the receiver output signal but also to collapse of frame synchronism and consequent long-continued code errors.
When the propagation delays are invariable with respect to time so that a phase difference between two route signals is constant, it is readily possible to avoid occurrence of such a code error by merely supplying one of the digital signal sequences derived from the two route signals to the digital signal combining circuit through a delay circuit. The technique of avoiding the code error is, however, not so simple when relative fluctuation may occur in the propagation delays of the two route signals or in the phase difference therebetween.
A baseband signal switching arrangement for diversity reception in a PCM radio communication signal is revealed in U.S. Pat. No. 4,015,205 issued to Kiyoshi Ikeda and Toshihiko Mitani, assignors to the present assignee. As will later be described more in detail with reference to one of several figures of the accompanying drawings, the arrangement comprises a plurality of buffer memories for temporarily memorizing the respective route digital signal sequence. Each buffer memory comprises, in turn, a predetermined number of memory cells. The receiver output signal is produced by reading one of the buffer memories that is indicated by the selection signal. A read-out signal for reading the selected buffer memory is synchronized with the route digital signal sequence memorized therein. The phase of the read-out signal therefore discontinuously varies when the selection signal selects another of the buffer memories that is for memorizing a digital signal sequence of a different phase. The arrangement is effective in eliminating the code error resulting from the relative fluctuation of the propagation delays. It is, however, impossible with this arrangement to eliminate the code error when the relative fluctuation in phase between two route signals from which the digital signal sequences to be switched from one to the other are derived, becomes at least equal to a half of the predetermined number in terms of clock period of the digital signal sequences. In other words, the disclosed arrangement needs buffer memories of a considerably large memory capacity.